For the precise determination of distances and/or changes in distance between reference points that are far apart, for instance in satellite geodesy or for determining the distance between two satellites, optical measuring methods are often used. In these, a measuring beam is normally emitted from one of the reference points, for example from the earth or one of the satellites, in the direction of the other reference point, which is associated with another satellite or another measuring object. The measuring beam is reflected at the measuring object and detected on its return to the first reference point. In the detection, a running time or a phase position of the returned beam is determined, for example, from which finally a distance or a change in distance between the reference points can be determined.
Due to the often great distances between the reference points, a precise back reflection of the measuring beam within the measuring system is necessary. At the same time, flying objects, such as satellites, as measuring objects are often exposed to random variations and rotary movements, which also affect a reflective device of the measuring system arranged in the flying object. The relatively great distance between the reference points here has the consequence that, when using a simple mirror as a reflective device, for example, even a slight tilting of the flying object can result in a large divergence of the reflected measuring beam. A measurement can be distorted or even become impossible due to this.
To avoid such influences due to tilting, retroreflectors are normally used as reflective devices. These have the property of reflecting incident light over a larger solid angle range precisely counter to its incoming direction in each case. In addition, in the case of retroreflectors in the form of triple mirrors in particular, the path length of a light beam within the retroreflector is independent of a lateral offset of the beam from an axis through the point of reference of the triple mirror. Corresponding movements of the measuring object do not affect the measuring result, therefore. The point of reference of the triple mirror thus corresponds at the same time to a stable reference point, which is associated with the flying object in the context of the optical measuring system.
Even when using a retroreflector, however, a rotation of the flying object can cause the reflective device to be moved along the axis of the measuring beam. The measuring system would therefore indicate a change of distance, although the position of the centre of gravity of the flying object may not have changed. In addition, in the case of a rotation of the flying object parallel to the connecting axis between the reference points, the reflective device can move laterally out of the range of the incident measuring beam. Both problems can be countered by leading the axis of the measuring beam at least approximately through the centre of gravity of the flying object. However, this considerably limits the options of an arrangement of the reflective device and thus also of the other installations in the flying object.
In some cases, for instance in the case of the GRACE Follow-On satellite mission, the central connecting axis between two satellites is additionally already occupied by other applications. A laterally offset arrangement of the optical measuring system with a measuring beam reflected into itself, i.e. a so-called monostatic structure, would result in the disadvantages described in the case of rotations of the satellite. A bistatic construction has therefore been proposed, in which the incoming and the reflected measuring beam run laterally offset from one another. The measuring beam impacts the measuring object here away from the main axis and is led back with the same offset on the opposite side of the main axis. The distance between incoming and reflected measuring beam is selected here so that the measuring beam circulates around the applications of the two satellites lying on the main axis as on a racetrack.
Even if the influences of variations are largely compensated by the guidance of the measuring beam described, a triple mirror of considerable size is required to achieve the required relatively large distance between incoming and reflected measuring beam. To save space and weight, it has therefore been proposed to reduce the large triple mirror of this kind to a few functional sections and to arrange these segments distributed in the satellite. This in turn calls for a high thermomechanical stability of the mirror segments in relation to one another, which can only be achieved expensively on account of their wide distribution.
Known monostatic constructions also have the further disadvantage that beam tracking in the case of rotations of the measuring object is only possible with difficulty, since a tilting mirror arranged for this purpose in the measuring path, for instance, would directly influence the measuring result. On the other hand, the alternatively proposed realignment of the overall measuring object can likewise only be implemented expensively, especially in the case of a satellite.
A reflective device that avoids the disadvantages is therefore desirable.